PROGRAM

Over the last 10-15 years, our understanding of the Earth's atmosphere and
its influence on astronomical observations has been dramatically improved. A
sound physical basis for the measurement and interpretation of the atmospheric
parameters of key importance for astronomical research at all wavelengths
has been established and translated into working field equipment. This
development has happened in increasingly close collaboration with
the meteorological and atmospheric sciences, drawing on atmospheric modeling,
and a better knoledge on turbulence parametrization.
As a result, the evaluation and intercomparison of existing and
potential observing sites has reached a level of sophistication, detail,
and reliability which was hardly dreamed of only 25 years ago.
The experience over this period has led to dramatic improvements in delivered
image quality at existing observatories, both with conventional telescope
optics in suitable mountings and enclosures and through the development of
adaptive optics and optical interferometry to the operational stage. These
advances greatly increase the demands on the atmospheric quality of potential
future sites for optical telescopes. At the same time, the wavelength range
covered from a single observatory has increased dramatically: At observatories
initially established for optical astronomy, optical and (sub)mm astronomy
have become of increasing importance. In addition, the existence of multi-year
cycles of climate variation makes it necessary to extend measurements over
long periods. Finally, diagnostics of the rapid rise in man-made perturbation
of the environment must also be included, such as potential airborne pollution
from (natural and) artificial sources, interference from urban development and
airline traffic, and radio noise from ground and space based radar or
communications installations. These and other factors must be quantified and
considered in the selection of future observatory sites.
Together, these developments greatly expand the range of atmospheric and other
environmental parameters to be explored when prospecting for potential
observatory sites. On the one hand, this implies greater complication and more
rational planning of the equipment and analysis techniques to be deployed in
future site testing campaigns. On the other hand, the more detailed
understanding of the atmosphere that results from these measurements is itself
of scientific value that transcends the narror borders of astronomy into the
environmental sciences.
Finally, the recent commissioning of several very large telescopes on a very
small number of sites has highlighted the need to prepare an inventory of the
- presumably few - remaining sites on Earth where adequate conditions still
exist for future major observatories, so that the options are known and
protective measures can be taken in time, if necessary.
We propose to hold an IAU Colloquium - the first on this subject for many
years - to review the state of the art in site testing methods and instruments
across the entire ground-based optical, infrared, and (sub)mm wavelength
ranges. Capabilities, commonalities, and complementarities between methods
will be clarified, with a view to establish lists of necessary and sufficient
equipment for site testing at various ambition levels as well as the
corresponding data reduction, calibration, and standardization procedures.
Links and synergisms between ground and space based techniques will be
explored with a view to mapping out a strategy for identifying additional
high-quality observatory sites, suitably distributed in geographical
longitude and latitude.